1. Field of the Invention
The present invention relates to ceramic articles and, in particular, high purity ceramic articles suitable for use as articles used to fabricate semiconductor materials.
2. Discussion of the Related Art
Although other materials may be used, e.g., Silicon-Germanium (SiGe) or Gallium Arsenide (GaAs), Silicon (Si) is presently the most important semiconductor for the electronics industry. Very Large Scale Integrated (VLSI) circuit technology (i.e., up to about 100,000 devices per chip), and Ultra Large Scale Integrated (ULSI) circuit technology (i.e., more than 100,000 and in some cases exceeding one billion devices per chip) are based almost entirely on silicon.
The fabrication of VLSI and ULSI circuits generally takes place on silicon substrates that possess very high crystalline perfection or purity. That is, in crystalline solids, the atoms which make up the solid are spatially arranged in a periodic fashion. If the periodic arrangement exists throughout the entire solid, the substance is defined as being formed of a single crystal. The periodic arrangement of the atoms in the crystal is called the lattice. Very high crystalline perfection requires that the silicon substrate possess a minimum of impurities and structural defects throughout its single crystal silicon lattice.
Generally, raw material, e.g., quartzite, is refined into electronic grade polysilicon (EGS) and melted. A silicon seed crystal is than used to grow a single crystal silicon ingot from the molten EGS. The ingot is then precisely sliced and polished into silicon wafers. The silicon wafers provide the substrates upon which VLSI and ULSI circuits are ultimately built through a complex sequence of wafer fabrication processes.
The increasing size of silicon wafers is one of the most obvious trends in silicon material technology. Presently, 300 mm diameter wafers are expected to ultimately replace most 150 mm and 200 mm wafer applications, and it is predicted that 400 mm wafers will be introduced in the near future. The use of larger diameter wafers presents several major challenges to semiconductor manufactures. For example, facilities with equipment capable of handling the larger wafers, e.g., vertical furnaces, must be built. New patterning techniques must be developed to print smaller feature sizes over larger areas. The larger wafers must also be thicker to increase their resistance to warping and other structural deformations. Moreover, the larger wafers are also heavier, requiring the use of automated wafer transport systems.
As the silicon wafers become bigger and heavier, the problem of preventing impurities and structural defects to the lattice, i.e., of maintaining very high crystalline perfection, becomes even more critical. Two such structural defects, which become especially problematic in 300 mm silicon wafers and larger, are “back side damage” and “slip” in the lattice structure.
Back side damage generally occurs when a wafer moves across a surface of a wafer support device, causing scratches in the back side of the wafer.
Slip in silicon wafers is a function of the stress applied to the wafer. This stress can be mechanical (e.g., frictionally induced) and/or thermal. As the wafers are stressed, the crystal lattice undergoes elastic deformation that disappears as the solid crystal returns to its original position upon release of the stress. However, severe stress leads to slip, which is the plastic or permanent deformation in the crystal lattice, which remains when the stress is released. Slip occurs when the elastic limit (or yield strength) of the silicon is exceeded and the lattice becomes permanently misaligned.
Slip is common during high temperature processing of silicon wafers in heat treatment furnaces (furnacing operations), as thermal stress is proportional to the processing temperature. The transition temperature from brittle to ductile behavior of the wafer is generally within the range of about 720 to 1000 degrees Celsius. Therefore slip, whether induced by thermal or mechanical stress, becomes especially problematic at process temperatures above 720 degrees Celsius.
Wafer carriers or boats are wafer support devices, which are subjected to furnacing operations during semiconductor wafer processing. Horizontal wafer boats are typically designed to support a horizontal row of wafers, which are inserted into a horizontal furnace tube for high temperature processing. Vertical wafer boats are typically designed to support a vertical stack of wafers, which are inserted into a vertical furnace tube. Generally, for large diameter silicon wafers, e.g., 300 mm, vertical wafer boats are more commonly used. This is because vertical furnaces have a smaller foot print than horizontal furnaces and therefore take up less of the expensive manufacturing space. Additionally, vertical furnaces generally demonstrate better temperature control than horizontal furnaces.
Wafer boats are generally composed of ceramic materials. Ceramic materials, which are joined by ionic or covalent bonds, are typically composed of complex compounds containing both metallic and non-metallic elements. Ceramics typically are hard, brittle, high melting point materials with low electrical and thermal conductivity, good chemical and thermal stability, and high compressive strengths. Examples of ceramic materials are quartz, silicon carbide (SiC) and recrystallized silicon carbide. One such recrystallized silicon carbide is available from Saint-Gobain Ceramics & Plastics, Inc., of Worcester, Mass., under the trade name CRYSTAR®. This material is a silicon carbide ceramic that has been impregnated with high purity silicon, and is also called siliconized silicon carbide. Conventional wafer boats formed from siliconized silicon carbide typically have a base or core of about 75% to 90% dense silicon carbide coated with silicon with a density of at least 99%. Referring to FIG. 1, a typical prior art vertical wafer boat 10 generally includes three or four support rods 12 extending vertically upwards from a generally circular horizontal base 14, and spaced radially along the periphery of the base. The rods 12 have a plurality of cantilevered wafer support arms (or teeth) 16 supported only at one end, which extend inwardly toward the center of the base 14 to define a series of slots therebetween. The slots are sized to receive the silicon wafers, which are supported by the arms 16 during furnacing operations.
Problematically for larger wafers, the prior art wafer support arms 16 provide most of their support at the outer periphery of the wafer. Accordingly, most of the weight of the wafer is unsupported and distributed toward its center. Therefore, during high temperature thermal processing, the center of the wafers tends to sag, promoting slip in the crystal lattice of the wafer.
Because of the geometry of the circular wafers, substantially half of the weight of the wafer, i.e., the inner wafer weight, is distributed within a circular area that is 70 percent the radius (R) of the wafer. Conversely, half of the weight of the wafer, i.e., the outer wafer weight, is distributed over a doughnut shaped area that has an inner radius of 0.7 R and an outer radius of 1.0 R. As a result, supporting the wafers at or about the 0.7 R circular boundary region of a wafer, e.g., from 0.6 R to 0.8 R, balances the inner and outer wafer weights and greatly reduces the potential for sagging during high temperature thermal processing.
Current prior art boat designs require deep slots, thereby making the arms 16 of the support rods 12 long enough to extend to the 0.7 R point. However, manufacturing this geometry is cumbersome due to the precise machining required and the inherently low yield rates. Also the added length of the cantilevered arms imposes a large moment force at the single support point where the arm attaches to the rod body, unduly increasing the probability of failure or breakage. Moreover, because the arms provide support at only three or four small discrete areas on the wafers, the possibility of back side damage is enhanced for the heavier wafers.
One prior art attempt to solve this problem was to provide a plurality of discrete circular ceramic rings having an inner radius smaller than 0.7 R and an outer radius greater than 0.7 R. The rings would be slid into each slot and the wafers would then rest upon each ceramic ring.
However, each ring typically costs in the range of one thousand to two thousand dollars to manufacture, greatly adding to the cost of the boat. Additionally, the rings essentially enclose the support area where the wafers rest. This makes it difficult for conventional transfer equipment to get between the rings and wafers in order to remove the wafers from the slots. Additionally, the enclosed design impedes the free flow of gasses that are often important to the processing of the wafers. Also, since the rings typically add up to 100 additional discrete moving parts to the boats (one for each slot), the potential for generating particles or other impurities is greatly enhanced. That is, the ceramic rings can rub against the wafer support arms during processing or handling, and frictionally rub off microscopic particles which can damage the semiconductor circuitry of the wafers.